1. Field of the Invention
This invention relates to composite ion exchange membranes useful in galvanic cells such as fuel cells.
2. Description of Related Prior Art
Sulfonic fluoropolymers containing ion exchange groups have been used widely in industry, particularly, as ion exchange membranes in chlor-alkali electrolytic cells and in other applications such as fuel cells and proton pumps. These membranes can be made from fluorinated polymers having ion exchange active groups or sites convertible to ion exchange active groups attached as pendent groups to the polymeric backbone. The fluoropolymers are usually thermoplastic and can be fabricated into films or sheets while in their molten form using mechanical extrusion equipment. Examples of multi-layer fluoropolymer film membranes prepared by molding the polymers are disclosed in U.S. Pat. Nos. 4,426,271; 4,123,336; 4,983,264; 4,544,471; and 4,411,750. Multi-layer composite film membranes can also be prepared by laminating individual films under heat and pressure as described in U.S. Pat. Nos. 4,253,923; 4,983,264; 4,544,471; and 4,411,750.
Such methods of fabrication can lead to problems of decomposition since molding, extrusion, and laminating mechanical equipment is usually operated in the temperature region near the crystalline melting point of the polymer which is commonly near the decomposition temperature of the polymer. Accordingly, there is disclosed in U.S. Pat. No. 4,784,882 a method of forming multi-layered composite ion exchange active fluoropolymer films suitable for use in electrolytic cells utilizing certain perhalogenated dispersants for use in forming dispersions from which films can be cast. The subsequent application of succeeding films is achieved subsequent to air drying and sintering the previously cast film.
For effective utilization of fluoropolymer ion exchange active film membranes, the physical integrity of the fluoropolymers is an important consideration. The physical integrity of ionic fluoropolymer films is determined largely by the amount of water or solvent the fluoropolymers contain. Thus, a sulfonic fluoropolymer that swells excessively because it absorbs substantial amounts of water or solvent tends to become gel-like and loose much of its physical integrity relative to an unswollen ionic fluoropolymer film. The level of swelling and the level of water absorption is largely determined by the temperature and the environment. For example, pure water at a given temperature will swell the ionic fluoropolymer more than aqueous salt-containing electrolytes which, in turn, swell the ionic fluoropolymers more than humid gases. However, increasing the temperature results in increased swelling and water absorption in each environment referred to above. Therefore, a single definition of suitable physical characteristics of an ionic fluoropolymer in order to define its usefulness is difficult to state since the utility of the ionic fluoropolymer film depends largely upon the environment in which it is used. Generally, the prior art workers have considered that electrolytic cell membranes for the electrolysis of sodium chloride are useful if characterized as having equivalent weights of about 800 to about 1500 as disclosed in U.S. Pat. Nos. 4,358,545; 4,417,969; and 4,478,695.
Other composite ion exchange membranes are disclosed in U.S. Pat. Nos. 4,337,137 and 5,246,792. In U.S. Pat. No. 5,246,792 a structure of an ion exchange composite membrane is disclosed in which an ion-conductive thin layer having a thickness of 5 microns is bonded to an ion exchange membrane having a thickness of 200 microns. The thin layer has a lower glass transition temperature during the bonding process than that of the membrane having a thick layer. The thickness of the thin layer can be between 0.01 to 20 microns. It is noted that the thick layer membrane of the composite when used as an ion exchange membrane in a fuel cell is arranged so as to face the cathode of the fuel cell.
U.S. Pat. No. 5,246,792 is directed, generally, to composite ion exchange membranes for use in an electrochemical fuel cell. Rather than define the composite membrane layers in terms of their equivalent weight, the layers are defined in terms of glass transition temperature. The glass transition temperature of the polymer forming the layers of the composite membrane is directly proportional to equivalent weight of the polymer. It is noteworthy that the teaching of the '792 patent with respect to orientation of a bilayer composite membrane is to place the thick layer, having a higher glass transition temperature, facing the cathode of the fuel cell. Such an orientation of the composite membrane is opposite to the orientation desired for the composite bilayer membrane of the invention which is oriented so as to obtain improved fuel cell potential by placing the lower equivalent weight layer facing the cathode.
In U.S. Pat. No. 4,337,137, a composite ion exchange membrane comprised of at least two layers is disclosed. The layers can have equivalent weights which differ from each other by less than 150. Useful equivalent weights for the membranes in chlor-alkali cells are not below about 800 to about 1100. It is noted that the abstract provides that the higher equivalent weight layer of the composite faces the cathode in an electrolytic cell so as to provide a barrier layer to back migration of hydroxyl ions.
Another factor in defining the usefulness of a sulfonic fluoropolymer as a membrane in an electrochemical cell for the production of chlorine and an alkali metal hydroxide is electrical conductivity and the ability to reject ions. Thus, a sulfonic fluoropolymer chosen for such use is usually selected based upon a balance between the electrical conductivity of the sulfonic fluoropolymer, which is affected by both the equivalent weight and the water absorption characteristics of the polymer and the ability of the sulfonic fluoropolymer to reject hydroxide ions, a property largely determined by the level of hydration of the polymer. That is, the degree of hydration per functional group in the sulfonic fluoropolymer. In order to minimize the passage of hydroxide ions, a sulfonic fluoropolymer is selected having a higher equivalent weight than would be required based merely upon electrical conductivity of the fluoropolymer alone. The selection of a sulfonic fluoropolymer is determined by the swelling characteristics of the polymer rather than the high ionic conductivity of the polymer, within the limitations of maintaining the physical integrity of the polymer. With respect to the use of sulfonic fluoropolymers in fuel cells and proton pumps, entirely different chemical and physical requirements for the membrane apply as opposed to selection of sulfonic fluoropolymers for use in chlor-alkali electrolytic cells. The different physical conditions present in a fuel cell result in different levels of swelling of the sulfonic fluoropolymer than result in chlor-alkali electrolytic cell environments. The hot electrolytes present in chlor-alkali electrolytic cells are not present in fuel cells, accordingly, there is little, if any, requirement for rejection of negative ions. The primary requirement for a membrane for use in a fuel cell is the transport of protons at the lowest possible electrical resistance. This makes a sulfonic fluoropolymer having the lowest equivalent weight consistent with the maintenance of physical integrity the best choice. Such low equivalent weight sulfonic fluoropolymers are disclosed in U.S. Pat. No. 4,940,525.
It is known to use sulfonic fluoropolymer membranes in proton exchange membrane fuel cells. In a proton exchange membrane fuel cell, an anode electrode is positioned in an anode compartment and a cathode electrode is positioned in a cathode compartment. The two compartments are separated by a membrane that is capable of transferring protons between the two electrodes. In a fuel cell, a reactant, for example, hydrogen, is fed to the anode and a reactant, for example, oxygen, is fed to the cathode. Reactions occur at the anode and at the cathode, thereby producing electricity. Protons pass through the membrane between the anode and the cathode. A membrane having the ability to transfer a large number of protons and be physically and chemically stable during the operation of the fuel cell has been shown to be a sulfonic fluoropolymer membrane such as a Nafion.RTM. fluoropolymer film, as disclosed by R. J. Lawrance in Interim Report New Membrane-Catalyst for Solid Polymer Electrolyte Systems, a report prepared for the University of California, Los Alamos National Laboratory.